TGSL Tuning & troubleshooting

[simonbaker]

The All Metal mode is really only a TR detector, but with ground exclusion and auto-tune. Therefore you must set up the ground balance for the ALL Metal Mode.

I don't know anything about Bandido, just TGSL. What is the definition of TR detector as you use it here?

It is possible that you are finding an alternative position for the pots where ferrite is being excluded, because both GEB and DISC channels are active during your tests. However, this would not be the correct ground balance position, and the ALL Metal mode would fail to exclude the ground effect, or even reject the ferrite.

Here's how I see it:

It's true that both channels are used if you listen to the audio beep. Of course if you just observe the output of the GB LM308 for ground balancing, then it doesn't matter what is going on in the DISC channel. And if you start with a disc setting that allows ferrite to beep, then it should be equivalent to ALL METAL mode anyway -- I don't see how it would affect where the GB Syncronous Detector shuts off the GB channel.

Because really the GB channel is just another DISC channel controlled by a separate sync pulse. The two channels are logically combined where the LM308 outputs go into the comparator. Changing to ALL METAL mode does not affect the GB channel at all. A target makes a beep when both channels move positive (output of LM308 ) simultaneously above the sensitivity threshold. So to ground balance, my understanding is you set the GB control just enough to keep ferrite targets under the sensitivity threshold in the GB channel. If you go more, you risk desensitizing or cutting off other targets as well.

So actually, if you set the DISC pot to hear the ferrite beep when you start, then only the GB pot affects where the signal disappears. It will disappear when when the GB channel signal falls below the comparator threshold.

Previously I asked:
what is the initial phase-shift of your coils, and what is the residual voltage after nulling

I would still like to know the answer.

I meant to answer that - I'll get you some data as soon as I hook everything back up. I had an upheaval here and had to move some things around.

I actually haven't settled on a phase shift, but I think I can get you the approx voltage.

I was trying a phase shift that makes voltages on C15 and C12 non-negative. Then you mentioned that the correct null phase is where the coils have more overlap, so I've been looking at that, since I like the idea of maximum overlap. But I recall that that phase seems to make more negative voltages on C15, C12 there.

In my "workshop" testing, I haven't found any null phase to perform superior to another. And as I've said, it doesn't seem to affect the DISC and GB calibration. But my bench is very crude and noisy and I don't want to jump to conclusions, those are just my findings, supported by the LTSpice sim and theories.

-SB

[simonbaker]

TR

Just had another ****flash****.

If by "TR detector" Qiaozhi means a "non-motion" mode, that might explain why we are not on the same paragraph.

TGSL does not have a "non-motion" mode at all. None. Doesn't exist.

I could imagine that a detector with a "non-motion" mode might have very strict requirements for the null phase, and that setting the null phase as Qiaozhi says could be important for a number of reasons.

So if Qiaozhi assumes TGSL uses a "non-motion" mode when ground balancing, that could explain our different views on how null phase influences GB and DISC calibration.

Just throwing that out...

-SB

[Qiaozhi]

Just had another ****flash****.

If by "TR detector" Qiaozhi means a "non-motion" mode, that might explain why we are not on the same paragraph.

TGSL does not have a "non-motion" mode at all. None. Doesn't exist.

I could imagine that a detector with a "non-motion" mode might have very strict requirements for the null phase, and that setting the null phase as Qiaozhi says could be important for a number of reasons.

So if Qiaozhi assumes TGSL uses a "non-motion" mode when ground balancing, that could explain our different views on how null phase influences GB and DISC calibration.

Just throwing that out...

-SB

You managed to read my post and start concocting a reply before I'd finished editing. If you look back at my post you will see there is no mention of the TR mode. After I wrote the post, and re-read it, I realised that the TGS is different to the Bandido, and does not have the TR (non-motion) channel. So you are correct. However, I cannot agree with your findings.

Let me suggest an experiment:
First set up the coil, and ground balance your detector using your method. Then re-adjust the coil overlap. You will see on the scope that the phase-shift between the signal at the TX and the signal across the RX coil changes as you adjust the overlap. Now position the coil overlap such that the phase-shift is in a different position. Is the detector stiil able to reject the ferrite ... without you having to re-adjust the GEB trimmer?

[simonbaker]

You managed to read my post and start concocting a reply before I'd finished editing. If you look back at my post you will see there is no mention of the TR mode. After I wrote the post, and re-read it, I realised that the TGS is different to the Bandido, and does not have the TR (non-motion) channel. So you are correct. However, I cannot agree with your findings.

Let me suggest an experiment:
First set up the coil, and ground balance your detector using your method. Then re-adjust the coil overlap. You will see on the scope that the phase-shift between the signal at the TX and the signal across the RX coil changes as you adjust the overlap. Now position the coil overlap such that the phase-shift is in a different position. Is the detector stiil able to reject the ferrite ... without you having to re-adjust the GEB trimmer?

You've just described (succinctly) the experiment I suggested!

And yes... that is what I find, the GEB doesn't need to be trimmed.

Actually, what I do is re-balance it each time and check the pot setting visually and note that it is in virtually the same position.

The reason I suggest rebalancing for each null is that we're looking for the minimum pot setting where the ferrite signal "disappears". Any pot setting beyond that will kill the ferrite signal also.

Of course there is some wiggle room in this because the ferrite signal starts to break up and is not great to begin with, so you'll never set it to exactly the same spot (even if you don't change the null). So you need to do the experiment a few times, keep an open mind, and I think the picture comes through.

The same principle holds for the discrimination control, and I find that easier to demonstrate. I find a US Nickel perfect for testing the discriminator. I set the null, note the pot setting to just knock out nickel, change the null, adjust again, and the pot setting is virtually the same both times.

-Concocter

[simonbaker]

To be more precise you should be monitoring the DC voltage on the output of the GEB sync demod (source of TR5).

(this may be an obsolete point by now, going back a few messages)

No, I very much disagree. It is the AC pulse generated by sweeping the target that you must make "go flat" to ground-balance the detector. The DC voltage is exactly what you don't look at (except to make sure it does not go below -.5 volts when you null).

You can probably see the AC pulse on source of TR5, but much easier at output of LM308.

Again, I'm speaking about TGSL which has no non-motion mode. With non-motion mode, you probably do want to adjust the DC voltage as you say.

Regards,

-SB

[Qiaozhi]

You've just described (succinctly) the experiment I suggested!

And yes... that is what I find, the GEB doesn't need to be trimmed.

Actually, what I do is re-balance it each time and check the pot setting visually and note that it is in virtually the same position.

The reason I suggest rebalancing for each null is that we're looking for the minimum pot setting where the ferrite signal "disappears". Any pot setting beyond that will kill the ferrite signal also.

Of course there is some wiggle room in this because the ferrite signal starts to break up and is not great to begin with, so you'll never set it to exactly the same spot (even if you don't change the null). So you need to do the experiment a few times, keep an open mind, and I think the picture comes through.

The same principle holds for the discrimination control, and I find that easier to demonstrate. I find a US Nickel perfect for testing the discriminator. I set the null, note the pot setting to just knock out nickel, change the null, adjust again, and the pot setting is virtually the same both times.

-Concocter

So what you are claiming is that the initial phase-shift in the RX coil is exactly the same wherever you adjust the overlap ... and this is quite simply not correct. Also, there would be no need for the GEB trimmer, as it could be replaced by a set of fixed resistors. Again, this is not the case. You need to look at the relationship between the TX and RX signals as you adjust the coil overlap. You will see quite clearly that the phase-shift changes with coil position. To make things clear - I am talking about a motion detector here, not a non-motion.

[simonbaker]

So what you are claiming is that the initial phase-shift in the RX coil is exactly the same wherever you adjust the overlap ... and this is quite simply not correct. Also, there would be no need for the GEB trimmer, as it could be replaced by a set of fixed resistors. Again, this is not the case. You need to look at the relationship between the TX and RX signals as you adjust the coil overlap. You will see quite clearly that the phase-shift changes with coil position. To make things clear - I am talking about a motion detector here, not a non-motion.

I love this...

Nope, I'm not claiming the initial phase shift of the RX signal is exactly the same wherever you adjust the overlap... I agree with you it changes radically.

I'm claiming that the ferrite target signal phase does not change when you adjust the overlap.

The ferrite target signal is added to whatever null signal you concoct with your overlap. The ferrite signal never changes phase (basically). And you never see the ferrite signal alone on your scope. But it is there.

And to ground balance the detector, you need to block the ferrite target signal alone. And you block the ferrite target signal by adjusting the GB sync pulse so it is centered on the ferrite target signal zero crossing (which is not the RX signal zero crossing). That ferrite target zero crossing does not change, regardless of your null signal phase. Yes the RX zero-crossing changes, but not the ferrite signal zero-crossing.

Let me ask: are you claiming that to ground balance the detector, you must center the sync pulse over the RX signal zero crossing that occurs when the ferrite is detected?

-SB

P.S. You did make a logical extension of my statements by saying you could replace the GB pot with fixed resistors. I am implying that to an extent. For standard built coils, the GB control will ground balance close to the same position, I believe about center of the pot. But no two coils receive a ferrite signal component of identical phase due to differences in construction, component variations, etc. so you still need some adjustment. However, the main reason you have a GB trimmer is to adjust to different ground conditions which are not the same as ferrite and have a different signal phase.

[Qiaozhi]

Now we're starting to get to the nub of the matter.

I love this...

Nope, I'm not claiming the initial phase shift of the RX signal is exactly the same wherever you adjust the overlap... I agree with you it changes radically.

Whew! That's what I've been trying to get you to say.

I'm claiming that the ferrite target signal phase does not change when you adjust the overlap.

I'll come back to this point in a minute.

The ferrite target signal is added to whatever null signal you concoct with your overlap. The ferrite signal never changes phase (basically). And you never see the ferrite signal alone on your scope. But it is there.

Agreed. It's just another metal target to the detector.

And to ground balance the detector, you need to block the ferrite target signal alone. And you block the ferrite target signal by adjusting the GB sync pulse so it is centered on the ferrite target signal zero crossing (which is not the RX signal zero crossing). That ferrite target zero crossing does not change, regardless of your null signal phase. Yes the RX zero-crossing changes, but not the ferrite signal zero-crossing.

Agreed, except for the part where you state that the "ferrite target zero crossing does not change." Now I have a couple of questions: Are you using a ferrite rod or a ferrite slug to do this, and how far away from the coil are you positioning the ferrite?

Let me ask: are you claiming that to ground balance the detector, you must center the sync pulse over the RX signal zero crossing that occurs when the ferrite is detected?
-SB

No, absolutely not. Read my parting comments at the end.

P.S. You did make a logical extension of my statements by saying you could replace the GB pot with fixed resistors. I am implying that to an extent. For standard built coils, the GB control will ground balance close to the same position, I believe about center of the pot. But no two coils receive a ferrite signal component of identical phase due to differences in construction, component variations, etc. so you still need some adjustment. However, the main reason you have a GB trimmer is to adjust to different ground conditions which are not the same as ferrite and have a different signal phase.

Agreed.

So, in conclusion:
I think we may be getting to the bottom of this.
What you appear to be claiming is that ground balancing the coil by using the ferrite technique always results in the GEB trimmer ending up in the same position, regardless of the initial phase-shift between the TX and RX coils after nulling. If so, then personally I cannot see how this could possibly be true. For example, if you overlap the coils such that the initial phase-shift is, say 50 degrees, then it's out of the range of the GEB trimmer. There is no way you could ground balance in this situation. This is why I have asked you to go through your ground balance procedure and measure the resultant initial phase shift and residual amplitude in the RX signal.

Let me explain further:
Ground balancing works by assuming that the ground signal only produces changes in amplitude but not in phase. Only real targets (both ferrous and non-ferrous) can produce a phase-shift. In that case (in theory, for ideal ground) there is only one place on the RX waveform that you can sample to eliminate the ground effect; and that is at the zero-crossing where amplitude changes alone will be ignored. Only phase changes can produce a signal. Therefore, when deciding where to position the overlap between the coils, you should adjust them such that the GEB sample pulse is positioned centrally over the zero-crossing of the RX signal, with the GEB trimmer in mid-position. You can simply do this by looking at the scope. Fix the coils at this point. Later, after the glue is set, make final adjustments with a small loop of wire while monitoring the DC level at the output of the GEB sync demod. In the correct position this DC voltage will be zero. This is the "factory" setup for a detector that has an external GEB pot. For a detector that has an internal trimmer (such as the original TGS) the final setup requires the use of a ferrite slug. Fisher, for instance, have what they call "the balancing wand", which is basically a wooden stick with a small ferrite slug embedded in the end. This adjustment is usually done in the static (non-motion / pinpointing) mode, but the TGS does not have this facility. However, the adjustment is still possible without any obvious problems. The idea is to adjust the GEB trimmer so that when the ferrite is moved across the search coil, there will be a weak positive response. This test should be done with the ferrite about 2 to 3 inches from the coil.
The reason for using a ferrite is because, unlike ideal ground, most real soil conditions do contain some slight mineralization, and the ferrite balancing moves the sample point slightly to one side of the zero-crossing point.
Because the TGS is also a non-motion detector, it can only produce an audio signal when the output of the GEB sync demod changes in amplitude, and this can only happen when there is an associated phase-shift in the RX signal. Ideal ground does not produce a phase-shift, hence no change is detected. In reality you cannot (for most soil conditions) simply set the GEB sample pulse at the zero-crossing. However, the position where the ferrite is rejected is very close to the original zero-crossing, within 5 degrees.

Perhaps you could also check the phase difference between the point where you are balancing with the ferrite and the original zero-crossing point (that is, when there is zero volts DC at the output of the GEB sync demod).

The bottom line is that the point where the ferrite is rejected is never far from the original zero-crossing. Therefore it is not possible for the GEB pot to always end up (after nulling with the ferrite) in the same position regardless of the coil nulling.

[lunamay]

hi
i was wondering...is it possible to make a sheeld for tx and rx (i mean faraday's cage) using a sheelding from a computer mouse or a keyboard

what do you think?
ps. i tried to make a sheeld using a kitchen al foil and eaven a al foli from capacitors but kitchen foil is too delicate and al foil from caps is very stiff

best regards

[Qiaozhi]

hi
i was wondering...is it possible to make a sheeld for tx and rx (i mean faraday's cage) using a sheelding from a computer mouse or a keyboard

what do you think?
ps. i tried to make a sheeld using a kitchen al foil and eaven a al foli from capacitors but kitchen foil is too delicate and al foil from caps is very stiff

best regards

The Faraday shield cannot be made of very thick material, otherwise it act as a target and desensitize the detector. Try using something like this ->
http://www.maplin.co.uk/Module.aspx?ModuleNo=4098

[lunamay]

but how about mouse wire which has good looking sheeld..can it be use to this project..how about kitchen foil?


best regards;

[Monk]

Now we're starting to get to the nub of the matter.

Originally Posted by simonbaker
I love this...

Nope, I'm not claiming the initial phase shift of the RX signal is exactly the same wherever you adjust the overlap... I agree with you it changes radically.

Whew! That's what I've been trying to get you to say.

I have yet to build a detector but I have done SPICE simulations of sections of the TGSL circuit. Why does the phase shift of RX vary with overlap? What parameter change causes this phase shift? I am referring to phase shift caused by nulling search coils.

I have modeled the search coils as a very loosely coupled set of inductors and varying K from 0 to .0001 There is the expected amplitude change between TX (L1) and L2 (RX), but I do not see (simulation) a phase shift. What am I missing in modeling the TX-RX pair?

Thank you in advance.

[dfbowers]

The Faraday shield cannot be made of very thick material, otherwise it act as a target and desensitize the detector. Try using something like this ->
http://www.maplin.co.uk/Module.aspx?ModuleNo=4098

Or get youself a survival blanket from a camping store.. cheap. and a lifteime supply of conductive mylar!! I have found that what I have used is conductive on one side only.

I have notice a difference in sensitivity with my HH2 coils.
Don

[Qiaozhi]

but how about mouse wire which has good looking sheeld..can it be use to this project..how about kitchen foil?


best regards;

I've never tried using mouse wire, but it sounds like an expensive option, unless of course you have a bunch of old mice just laying around.
Personally I don't like using kitchen foil, but it is possible. Make sure you don't make too much of an overlap.
The adhesive aluminum tape is very cheap and simple to apply.

[Qiaozhi]

I have yet to build a detector but I have done SPICE simulations of sections of the TGSL circuit. Why does the phase shift of RX vary with overlap? What parameter change causes this phase shift? I am referring to phase shift caused by nulling search coils.

I have modeled the search coils as a very loosely coupled set of inductors and varying K from 0 to .0001 There is the expected amplitude change between TX (L1) and L2 (RX), but I do not see (simulation) a phase shift. What am I missing in modeling the TX-RX pair?

Thank you in advance.

That's a really good question, and I'm not sure of the answer. The coupling coefficient (as you've discovered) only affects the amplitude. I've not really thought about it that much, as I usually leave out the TX circuit altogether when simulating the RX circuits. If you include the TX oscillator it introduces a large overhead in simulation time, mainly because you have to constrain the simulator by setting the maximum allowed time step, and there is not really any benefit to be gained by including it anyway.
The problem with attempting to simulate the search coil is the lack of a suitable model. Using ideal inductors and coupling coefficients is probably not the best approach. However, it would be quite simple to put together a subcircuit where the TX-RX phase-shift was passed in as a parameter. I suppose that could be useful if you were trying to simulate the whole design, but you could just as easily generate the RX signal using a controlled source.